Many polymers and thin films on semiconductors are designed with specific molecular orientation to take advantage of mechanical, chemical, or physical properties. Some plastics, for example, exhibit different mechanical strengths depending on the orientation. Molecular orientation can be extracted from polarized ATR measurements.
This application note demonstrates determining the correct orientation of the polarizer for these measurements and illustrates the importance of polarized measurements in specific examples.
Experimental
For all measurements, a Harrick Wire Grid Polarizer (KRS-5 substrate) was used. The polarizer was oriented in the same position for both background and sample measurements. All spectra were collected using an FT-IR spectrometer at an 8 cm-1 resolution, averaging over 32 scans.
Harrick’s Horizon, Seagull, and VariGATR accessories were used here. Such accessories redirect the infrared to and from the sample via mirrors, changing the direction of the electric field, which makes it tricky to ascertain the polarizer position for s- or p-polarization. For ATR, the polarizer orientation is straightforward to determine. The effective thickness for p-polarization is nearly twice that for s-polarization at a 45° incident angle (1), which is directly reflected in band intensity.
Three samples were investigated. An unoriented liquid, water, was used to demonstrate the method used to determine polarizer orientation with the Horizontal multiple reflection ATR. Then, two oriented samples were examined: Tunicin was measured with the Seagull at a 45° incident angle and an oriented thin film on silicon was examined with the VariGATR at a 65° grazing incidence.
Results and Discussion
Figure 1 shows the ATR spectra of water, measured with the two perpendicular orientations of the polarizer. Clearly, the 0 setting gives bands that are roughly twice as strong as those for the 90 setting, indicating that the former gives p-polarized radiation.
Figures 2 and 3 show spectroscopic differences resulting from polarized measurements. In Figure 2, the O-H and C-H stretching regions change with polarization, suggesting different orientations of functional groups on the surface. Figure 3 shows differences at 1500 cm-1 and 1250 cm-1. These bands are not visible with s-polarization but are with p-polarization, which probes bonds perpendicular to the interface. This shows the preferential orientation of functional groups on the surface.
Conclusion
Combining a polarizer with ATR measurements provides valuable information about the sample. It also determines that the polarization delivered is straightforward. Polarization can enhance band intensities and provide information on oriented functional groups.
References
Harrick Scientific Products, Inc.
141 Tompkins Ave., 2nd Floor, Pleasantville, NY 10570
tel. (800) 248-3847
Website: www.harricksci.com
Using Thermostable Raman Interaction Profiling (TRIP) For Protein Binding Screening
March 1st 2024Narangerel Altangerel, Zhenhuan Yi, and Marlan Scully of Texas A&M University recently used TRIP to analyze eight protein–ligand systems. Spectroscopy recently spoke to these three researchers about their findings and what the implications are for high-throughput drug screening.
Reviewing the Impact of Raman Spectroscopy on Crop Quality Assessment: An Interview with Miri Park
February 1st 2024Miri Park of the Fraunhofer Institute for Environmental, Safety, and Energy Technologies is examining how Raman spectroscopy could aid non-destructive sensing in agricultural science. Recently, Park sat down with Spectroscopy to discuss micro-Raman spectroscopy's role in assessing crop quality, particularly secondary metabolites, across different contexts (in vitro, in vivo, and in situ), while suggesting future research for broader application possibilities.